Girls get their period at a range of different ages, but usually somewhere in their teens. On average, girls are abour 12 to 13 years old when they get their first period, however sometimes girls will get it earlier or much later. Either way, it usually doesn't matter. Typically a girl will get her first period two to two and a half years after breast development begins.
However, if a girl has any of the following warning signs, then she should see her doctor to discuss and have further investigations:
*No menstrual period within three years of breast development.
*No menstrual period by age 13, with no signs of pubertal development.
*No menstrual period by 14, with signs of excess facial or body hair (hirsutism).
*No menstrual period by 14, with a history or examination suggestive of excessive exercise or an eating disorder.
*No menstrual period by 14, with concerns about genital tract problems.
*No menstrual flow by 15.
*Menstrual periods that become "markedly irregular" after happening regularly every month.
*Menstrual periods occurring more than every 21 days.
*Menstrual periods occurring less than every 45 days.
*Menstrual periods occurring 90 days apart, even for one cycle.
*Menstrual periods that last more than seven days.
*Menstrual periods requiring frequent pad/tampon changes (more than once every one or two hours).
WebMD, Menstruation: What's Normal for Girls (www.webmd.com/parenting/news/2006 ...)